restore single exit point in parse_xrefs

It turns out that this function was in fact meant to always assign a
result (NULL/0 on failure), accomplished by having a single exit point.
This was changed in 517b81ad for no reason. Reverting.

I'm guessing the goto was considered disagreeable, so I'll explain the
rationale. The function accumulates its result in the *local* variables
xrefs and n. This mainly makes the code nicer to read than writing to
the output directly. Having a single exit point, a property that is easy
to verify, ensures that no update to the local variables can get lost,
i.e. they serve as de-facto aliases for the outputs.